76 Tradewind totally gutted. Went inside AS in middle of strong rain storm. Surprisingly only 1 leak where old ac is but both tail lights had leaks. When rain stopped drilled out entire assembly, 3 little things that looked like rivet heads. Assembly came off and is very heavy. Parts for it are not a problem. HOW DO YOU INSTALL ASSEMBLY back against the trailer. No perforations, no holes. Totally lost.
Assembly looks like cast "aluminum" or something like it

On my '73, those assemblies are riveted from the inside of the trailer. You have to have the inner skins off to see it. Strange way to do it. I only know this because one of mine actually fell off a while back. I had to screw it back on from the outside with stainless steel screws. I think it's in a thread someplace. See if I can find.